not sure how old you are but continue swimmimg .Why? because running will eventually take it out of your knees and most runners i see who run who are older say 50 plus, always look as though they really dont want to do it but feel they have to.
if youve swam all your life , ive no need to tell you that its strength building, ie an anaerobic as well as aerobic exercise.The water provides support and is often recommended for those with arthritic conditions.For all round conditioning , i dont think you can go far wrong with swimming